Arabian and Half-Arabian horses come in an array of coat colors. The genetics behind these colors are fascinating, as well as important for those who value coat color as a trait in their breeding program. Every horse, Arabian or otherwise, has a base coat color of either black or red. This base color can be modified by a series of other genes to determine the actual physical color of the horse’s coat. Interestingly, all Arabians, have black skin, except under white markings, regardless of their coat color. This black skin would be useful to provide protection from the hot desert sun of their heritage.
Black and Red
Every horse has two copies of either the black (E) or the red (e) gene; one copy is inherited from each of the parents. The red gene is recessive, meaning there must be two red genes for it to be expressed. So, all chestnut horses are (ee), or homozygous, meaning they have two copies of the same gene.
Black horses may be either homozygous black (EE) - with two copies of the black gene- or heterozygous black (Ee) - meaning they have one copy of each gene. Black is one of the least common coat colors in Arabian horses, as it’s often suppressed by the dominant Agouti gene.
The Agouti gene (A) is responsible for bay coat coloring and is visible only on black. Agouti causes depigmentation of black hair leaving a more reddish (bay) color, restricting the base black color out to points such as the legs, mane, tail, and tips of the ears. A horse with a red base coat (ee), can carry the Agouti gene, but it will not be expressed in physical appearance as the coat is already red. Agouti is a dominant gene, meaning a horse with one copy (Aa) will always express the color provided they also carry black.
The gray gene is dominant, and when present will always express itself over others. A horse that is heterozygous, or has one copy of the gray gene (Gg), will always be gray, but may produce offspring of other colors. A homozygous gray horse (GG) will always produce gray offspring, as all get will receive a dominant gray gene.
While physically a horse’s coat can be gray in color, genetically gray is not a color at all but a modifier. All gray horses have a base coat color of either black or red. The presence of the gray gene means as they age they undergo rapid depigmentation of this base color and will eventually become white. This explains why gray horses are often born chestnut or bay, and gradually fade to gray as they age.
Horses gray in different ways, some tuning rose or steel gray, while others very quickly become porcelain white. Gray horses can also have interesting patterns in their coat such as bloodmarks. They can also become fleabitten as depigmentation begins to reverse in places on their body.
The Arabian Horse Association recognizes bay, gray, chestnut, black and roan as coat colors for Purebred Arabian horses. Though Purebreds do not carry dilution genes such as cream or dun, they can be crossed successfully with other breeds to produce Half-Arabian horses in an array of colors. Unusual coat patterns such as sabino, dominant white, and rabicano are also seen in some horses, both Purebred and Half-Arabian.
Photos provided by April Visel, Nancy Pierce and Wojtek Kwiatkowski